Abnormality diagnosis apparatus for NOx sensor

Abstract

An NOx catalyst of storage reduction type and an after-catalyst NOx sensor are provided in an exhaust passage in an internal combustion engine. The after-catalyst NOx sensor detects the NOx concentration on the downstream side of the NOx catalyst. The NOx concentration on the upstream side of the NOx catalyst is detected or estimated. The possible abnormality of the after-catalyst NOx sensor is determined by comparing the NOx concentration on the upstream side of the NOx catalyst detected or estimated and the NOx concentration detected by the after-catalyst NOx sensor, under the condition that the NOx catalyst does not substantially absorb NOx in exhaust gas. Abnormality diagnosis is performed with the possible impact of the NOx catalyst inhibited.

1. An abnormality diagnosis apparatus for an NOx sensor comprising:
 an NOx catalyst of storage reduction type provided in an exhaust passage in an internal combustion engine; 
 an after-catalyst NOx sensor detecting NOx concentration of exhaust gas on a downstream side of the NOx catalyst, 
 before-catalyst NOx concentration acquiring means for detecting or estimating the NOx concentration of the exhaust gas on an upstream side of the NOx catalyst; 
 catalyst temperature acquiring means for detecting or estimating temperature of the NOx catalyst; 
 abnormality determining means for determining possible abnormality of the after-catalyst NOx sensor by comparing the NOx concentration detected by the after-catalyst NOx sensor and the NOx concentration detected or estimated by the before-catalyst NOx concentration acquiring means, under a condition that the catalyst temperature detected or estimated by the catalyst temperature acquiring means is such that at the temperature, the NOx catalyst does not substantially absorb the NOx in the exhaust gas; and 
 rich spike control means for performing rich spike control for allowing NOx absorbed by the NOx catalyst to be released, and performing the rich spike control before the after-catalyst NOx sensor detects the NOx concentration. 
 
     
     
       2. The abnormality diagnosis apparatus for the NOx sensor according to  claim 1 , wherein the before-catalyst NOx concentration acquiring means comprises at least one of estimation means for estimating the NOx concentration of the exhaust gas emitted by the internal combustion engine based on an operating condition of the internal combustion engine and a before-catalyst NOx sensor detecting the NOx concentration of the exhaust gas on the upstream side of the NOx catalyst. 
     
     
       3. The abnormality diagnosis apparatus for the NOx sensor according to  claim 2 , wherein the before-catalyst NOx concentration acquiring means comprises both the estimating means and the before-catalyst NOx sensor, and
 the abnormality determining means compares the value of the NOx concentration detected by the after-catalyst NOx sensor, the value of the NOx concentration detected by the before-catalyst NOx sensor, and the value of the NOx concentration estimated by the estimation means to determine the possible abnormality of the after-catalyst NOx sensor and the before-catalyst NOx sensor in a distinguishable manner. 
 
     
     
       4. The abnormality diagnosis apparatus for the NOx sensor according to  claim 1 , wherein the abnormality determining means determines the possible abnormality of the after-catalyst NOx sensor based on the NOx concentration detected by the after-catalyst NOx sensor under a condition that the after-catalyst NOx sensor is active.